1. Field of the Invention
The present invention relates to a magneto-optical recording apparatus which is capable of both magnetic modulation type, and light modulation type, recording.
2. Description of the Related Art
Various magneto-optical recording apparatus of the light modulation type are known and available on the market. This type of magneto-optical recording apparatus performs recording of an information signal on a magneto-optical recording medium, such as a disk, by the irradiation of a portion of the magneto-optical disk with a light beam whose intensity has been modulated in accordance with the information signal and by the application of an external magnetic field to the portion the magneto-optical disk which is being irradiated with the light. FIG. 1 illustrates the recording principle of such a light modulation type recording apparatus. It is assumed that a magnetic film 100b of a magneto-optical disk 100 is initialized in a downwardly magnetized state. A laser light source 101 generates a beam of light for recording information. On and off of the laser light source 101 is controlled based on an information signal input to a drive circuit 102. Drive circuit 102 drives the laser light source 101. The laser beam is condensed onto the magnetic film 100b by means of an optical system 103. Once the laser beam is irradiated onto the magnetic film, the temperature of the portion (a spot) of the magnetic film 100b which is irradiated with the laser beam rises to the Curie point, or to a temperature above which magnetization of the film can be readily reversed. A bias magnet 104 generates a constant magnetic field B on the surface of the disk when supplied with a direct constant current by means of a constant current source 105. Hence, only the portion of the magnetic film 100b which is irradiated with the laser beam is magnetized in a direction opposite to the initialized direction, i.e., in an upward direction. The magnetized state of that portion is preserved by the passage of that portion through the laser beam irradiation position and by the resultant decrease in the temperature thereof. Information signals are recorded on the magnetic film 100b in a pattern of magnetized areas, each of which has substantially the same size as that of the laser spot (about 1 .mu.m), by turning on and off the laser beam. In FIG. 1, reference numerals 100a and 100c respectively denote a protective film for protecting the magnetic film 100b and a glass or plastic substrate. However, the protective film 100a of the disks presently available on the market is not designed to withstand any impact. Therefore, such disks are not suitably used with magnetic field modulation type recording devices, discussed herein.
The information signals recorded on the magneto-optical disk are reproduced by utilizing the interaction between the light and magnetic force, (known as the magnetic Kerr effect) that is, by irradiating the magnetic film with a laser beam having a fixed intensity and then by detecting rotation of the polarization plane of the laser beam reflected by the magnetic film.
The magnetic Kerr effect is a phenomenon in which the plane of polarization of a linearly polarized laser beam, which is reflected by a film magnetized in a direction perpendicular to the direction of propagation of the laser beam, rotates in either clockwise or counterclockwise direction, depending on the direction of the magnetization. for reproduction of an information signal, this rotation is converted into light intensity by means of an analyzer.
When the information signal recorded in this type of recording medium is to be replaced with another one, the magnetic film 100b is uniformly magnetized in a downward direction by reversing the direction of the magnetic field generated by the bias magnet 104 (by making it downward) and by continuously irradiating the magnetic film with a non-modulated laser beam to erase the previously written information signal. Thereafter, a new information signal is recorded using the above-described procedure.
In other words, in the light modulation type recording apparatus now available on the market, it is impossible to write an information signal directly on another information signal already recorded on the disk.
Recently, a magnetic field modulation type recording apparatus, in which an information signal can be written directly on another information signal, has been proposed in, for example, the Japanese Journal of Applied Physics, Vol. 26 (1987) Supplement 26-4, "High Speed Overwritable Magneto-Optic Recording." FIG. 2 illustrates the recording principle of this type of recording apparatus. In FIG. 2, similar components to those in FIG. 1 are indicated by like reference numerals. A laser light source 101 is supplied with a d.c. current by a constant current source 106 to continuously emit a laser beam which is condensed onto magnetic film 100b by means of optical system 103. The temperature of the portion (spot) of the magnetic film which is irradiated with the laser beam rises to the Curie temperature, or to a temperature above which the direction of the magnetization can be readily reversed. A magnetic head 108 provided on a floating type slider 117 is held at a position 10 .mu.m or less above the surface of the disk 100 during recording. To prevent damage to the magnetic film 100b caused by contact of the floating slider 117 with the surface of the disk, or by dust disposed between the slider and the surface of the disk, a special protective film 100a, in which a filler or the like is present is formed on the magnetic film 100b. Because of this special protective film, the disks employed in such a magnetic field modulation type recording apparatus exhibit better durability than those employed in the light modulation type recording apparatus. The magnetic head 108 is driven by a magnetic head drive circuit 109 to generate a magnetic field whose direction is reversed in accordance with an input information signal. Recording of the information signal at a high speed requires the provision of a small magnetic head very close to the disk. Hence, the provision of the magnetic head on the floating type slider is desired in the magnetic field modulation type recording apparatus, unlike in the light modulation type recording apparatus.
Only the portion of the magnetic film 100b which is irradiated with the laser beam is heated and magnetized in the same direction as the direction of the magnetic field generated by the magnetic head 108. The temperature of that portion rapidly decreases after the rotation of the disk causes that portion to pass through the laser beam irradiation position, by which magnetization thereof is preserved. Information signals are recorded on the magnetic film 100b in a pattern of magnetized areas, each of which has substantially the same size as that of the laser spot (about 1 .mu.m), by reversing the magnetic field generated by the magnetic head. In the magnetic field modulation type recording apparatus, it is not necessary to make uniform the direction in which the magnetic film is magnetized, i.e., to perform an erasure operation, when the information signal recorded on the disk is replaced with new a one. Rather, it is possible to write an information signal directly over an already recorded information signal a desired number of times by repeating the above-described operation. Reproduction of an information signal is performed in exactly the same manner as in the light modulation type recording apparatus.
Thus, the light modulation type magneto-optical recording apparatus, which is not capable of writing an information signal directly over another information signal, which is available on the market, and the magnetic field modulation type magneto-optical recording apparatus, which has been proposed to write an information signal directly over an already recorded information signal, differ from each other in the protective film of the disks used as the magneto-optical recording media, and in the configuration of the magnetic field application means. Thus, the two are not at all compatible with each other. In particular, the effective range of the magnetic field generated by the magnetic field modulation type magnetic head is as small as 0.2 mm in diameter, and a positioning accuracy of approximately 0.05 mm is thus required between the magnetic field and the laser beam spot formed on the disk. In a case when a large light modulation type bias magnet and a small magnetic field modulation type magnetic head are separately provided on the same recording apparatus, the position of both the bias magnet and the magnetic head is mechanically switched over by a moving mechanism that is large and whose size increases, depending on the type of disk employed. However, a moving mechanism that is not large as to move the bias magnet and the magnetic head on a horizontal plane relative to the surface of the disk cannot assure the required positioning accuracy of 0.05 mm between the laser spot and the magnetic field modulation type magnetic head. Furthermore, the provision of the light modulation type bias magnet and the magnetic field modulation type magnetic head as separate components increases the overall size of the apparatus.